This invention relates to a method of forming images of sensor patterns on a photosensitive member for effecting image density control in an electrophotographic copying apparatus.
In one method known in the art of controlling the density of an image of a copy of a document produced by means of an electrophotographic copying apparatus to obtain the desired degree of density for the reproduced image, a sensor pattern of a reference density is provided substantially flush with the document support surface and irradiated from a document image exposing light source to expose a photosensitive member to an optical image of the sensor pattern by means of an exposing optical system to form thereon a latent image of the pattern which is developed into a visible image of the pattern by means of an image developing device. The visible image of the pattern thus obtained is sensed by means of photoelectric element to produce an electric output which is used for effecting control of the toner density of the developing agent, the amounts of exposure and the values of developing bias, and charging conditions that influence the density of the visible image, to thereby effect control of image density. This method is generally referred to as a P-sensor method and the photoelectric sensor used in this method is referred to as a P-sensor element while the sensor pattern is referred to as a P-sensor pattern.
The value of an output of the P-sensor element is inputted to a control circuit where it is compared with a value set beforehand for a standard image density. When they differ from each other, the toner density, the amount of exposing, the value of the developing bias, or charging conditions are revised in accordance with the difference between the output of the P-sensor element and the standard image density, so that control is effected in a manner to allow copies of the standard image density to be obtained in a stable manner when the document to be copied has the standard image density.
When there is a difference between the density of the document and the standard image density or when a document of the standard image density is used to produce copies of images of higher or lower density than the standard image density, the value of a density set for controlling the image density compared with an actual output of the P-sensor element may be varied to thereby vary the conditions of charging, exposing and developing, thereby enabling copies of the desired image density to be obtained.
Images of patterns formed on the photosensitive member that are used in this control method are obtained by the same means and following the same process as those used for forming images of a document on the photosensitive member. Thus the method is suitable for use as a method for controlling image density. However, unless a sensor pattern is mounted in a right place and has a high dimensional accuracy, variations would occur in the position of an image of a toner pattern formed on the photosensitive member and its dimensions would deviate from the actual dimensions of the sensor pattern. When copying is performed in an unequal image size copying mode, an image of the toner pattern would also be formed in the unequal image size copying mode so that it would be reduced or enlarged as the case may be. When this is the case, an error would be produced and an unnecessary load would be applied to the cleaning device, if sensing of the density of the image of the toner pattern is performed by means of a photoelectric element located in a predetermined position.
In effecting pattern image formation in the prior art, it has hitherto been customary to scan a sensor pattern by means of a scanner (scanning and exposing member) at the same scanning velocity as a scanning velocity at which scanning of the document image regions would be performed. The photosensitive member has hitherto been moved at a predetermined velocity to effect exposing of the photosensitive member to an optical image of the sensor pattern under the same conditions as the conditions under which exposing of the photosensitive member to a document image would be performed. When pattern formation is carried out as described hereinabove, it is necessary to bring the scanner to a halt with a high degree of accuracy in a predetermined position, making it necessary to effect assembling of parts and adjustments thereof with great care. To move the sensor pattern at a predetermined velocity would require the use of a section for preliminarily moving it at a lower velocity before it achieves the predetermined velocity, and the distance to be covered by the scanner in its movement would have to be greater than the required distance for effecting scanning. This would increase the size of the copying apparatus.